The present invention relates to an antifriction bearing for use in an environment involving vibration or impact, and also to an alternator incorporating the bearing for use in vehicles.
The bearing rings and rolling members of the antifriction bearing (hereinafter referred to simply as the "bearing") are generally subjected to a cyclic high-shear stress due to a rolling motion, so that they are given high Rockwell hardness of HRC 58 to 64 by hardening and tempering so as to retain increased strength against rolling fatigue. It has been reported that there is the following relationship between the hardness and the rolling fatigue life. The life greatly shortens as the hardness decreases. EQU LH=fH.sup.p .multidot.L
wherein
LH: the life when the hardness varies PA1 fH: the hardness factor PA1 p: a constant (3 for ball bearings, or 10/3 for roller bearings) PA1 L: the life of the standard bearing Further, PA1 fH=(HV/750).sup.2 PA1 (1) The surface of the steel in contact with the roller underwent plastic deformation due to load stress. The width of contact therefore increased to result in a lower contact surface pressure consequently improving the pitting life. PA1 (2) Rotation bending fatigue test revealed that the specimen containing 30 to 50% .UPSILON.R (residual austenite) was about 2 times the specimen of pure martensite in fatigue strength improvement. PA1 (3) 14NiCr14 specimen with 50% .UPSILON.R was HV 550 in hardness. The testing changed the surface hardness to HV 950. PA1 (4) It was not apparent whether the testing converted the .UPSILON.R to martensite. After the testing, a carbide was observed in the structure microscopically.
wherein HV is Vickers hardness
Recently, however bearings are used for applications wherein the intended rolling fatigue life is not available merely by assuring the hardness.
With common bearings, the shearing stress due to the contact between the bearing rings and the rolling members develops a crack from an inclusion or the like, and the crack grows to cause flaking. In the case where the inner ring is rotated, such flaking occurs predominantly in the rotating ring, i.e., the inner ring. Conversely, in the case of bearings which are used in an environment involving vibration or impact, the vibration or impact causes many minute cracks or changes in the structure immediately under the raceway of the outer ring which is fixed, consequently giving rise to flaking within a very short period of time to render the bearing unserviceable.
This phenomenon appears attributable to the following reason. The vibration or impact deforms the raceway and becomes more pronounced, consequently causing greater microscropic strain in the ring under the raceway.
The rolling fatigue life can be lengthened most easily by increasing the size of the bearing to give an increased load capacity. This achieves an advantage since a reduced stress value will then result when the same load is applied. In actual use, however, the vibration or impact load readily varies with the structure around the bearing and mounting and operation conditions, and it is impossible to meet the requirement of decreasing the size and weight, so that the increase in the size of the bearing is not a satisfactory solution.
High-carbon chrominum steel (such as JIS SUJ2 or SAE 52100) as adjusted to -the hardness of HRC 58 to 64 by the usual hardening and tempering treatments as stated above is conventionally used for the inner and ouster rings of the bearings for alternators for vehicles.
In recent years, however, alternators are required to have a smaller size, reduced weight and higher output to meet the need to decrease the fuel cost of vehicles and increase various electrical loads thereof. To fulfill this requirement, it has become practice to use a greater pulley ratio and to rotate the alternator at a high speed. Accordingly, the maximum speed of rotation is in excess of 12000 r.p.m.
The problems involved in the high-speed rotation include the slippage of the belt in connection with the external arrangement of the alternator. This problem has been overcome by using a larger number of belts under increased tension. On the other hand, the problem associated with the internal arrangement of the alternator is the need to render the bearing resistant to the high-speed rotation and to the high tension involved. More specifically, because the heat of agitation due to the high-speed rotation and the increased frictional heat due to high tension shorten the life of the grease used, the bearing must be adapted to overcome this problem. Furthermore, the bearing must be rotatable at a high speed without marked vibration that would result from the deformation of the raceway due to high tension. Generally, the bearing is rotatable at a high speed satisfactorily when reduced in size, since the side reduction is effective for decreasing the amount of heat generation.
Nevertheless, in the case where the bearing is subjected to high tension as in the alternator, a reduced size leads to a decreased load capacity to entail a shortened fatigue life, so that the bearings in use are at least about 32 mm in outside diameter if smallest.
Briefly, in assuring high-speed rotation under increased tension as required for reducing the size and weight of the alternator and increasing the output thereof, it is necessary to solve the conflicting problems of inhibiting heat generation and taking a countermeasure against the increased load while diminishing the vibration, whereas difficulties are encountered with the convention alternator described in overcoming these problems.